The invention relates to a method for drilling geothermal wells whereby the raw steam produced is treated with ferric chelates to remove substantially all of the hydrogen sulfide gas contained therein. The invention also relates to a stabilized ferric chelation system, and particularly to the use of stabilizers that increase the life of the iron chelating agents in removing hydrogen sulfide from geothermal steam produced during drilling operations.
Geothermal steam wells are drilled with compressed air as the motive force for the drill bit. Due to the high temperatures encountered and the highly fractured nature of the reservoirs, drilling mud is not an option. As the drilling enters steam producing areas, the air and steam escape through the casing into a blooie line and muffler. The stringent air pollution standards require that the H.sub.2 S in the steam be abated.
Once a well is completed, a flow test is performed. A flow test consists of opening the well full bore and letting it blow through the blooie line. It is during this period that a majority of the need for H.sub.2 S abatement occurs. These flow tests last from 30 up to 90 days depending on the experience with the reservoir.
Once wells are in production, they tend to foul as a result of the impurities present in the steam. Eventually, production will drop off and reworking of the well is necessary. Reworking involves bringing a rig in and drilling deeper into another steam producing zone or widening the bore. Again, H.sub.2 S abatement is necessary. As an area of steam production is reached, the well is again flow tested. The flow test at this point is much less, from 15 to 30 days. Generally, H.sub.2 S abatement requirements for a reworked well are about half that of a new well.
It is known from U.S. Pat. No. 4,151,260 that H.sub.2 S gases contained in steam can be abated during the drilling of geothermal wells by treating the steam with an alkaline solution of hydrogen peroxide.
The use of iron chelates to treat exhaust geothermal steam from a steam turbine is taught in U.S. Pat. Nos. 4,414,817 and 4,468,929.
It is known from U.S. Pat. No. 4,629,608 that cationic polymeric catalysts are useful to accelerate the oxidation of H.sub.2 S with iron chelates in exhaust steam from geothermal power plants. It is disclosed in U.S. Pat. No. 4,696,802 to treat steam released during geothermal well drilling with an aqueous solution of ferric chelate and a water soluble cationic polymeric catalyst to remove hydrogen sulfide therefrom.
It is known from U.S. Pat. No. 4,009,251 to use a metal chelate catalyst solution for hydrogen sulfide removal which contains a salt of a non-oxidizing acid having a pK of 1.2-6, such as formic and benzoic acids, to inhibit the formation of acidic sulfur oxides and accelerate the reaction of hydrogen sulfide to form sulfur.
The Stretford process is a well-known method for removing hydrogen sulfide from gas streams. Typically, the Stretford process uses a solution containing quinone sulfonate catalyst, a polyvalent metal, usually vanadium, and a carboxylate. The quinone is generally required in such systems to solubilize the polyvalent metal and/or the polyvalent metal carboxylate complex. As far as applicant is aware, however, such solubilizers are not required in systems employing polyamino carboxylate late chelating agents. Exemplary references descriptive of the Stretford process include U.S. Pat. Nos. 4,009,251; 4,243,648; and 3,937,795.
It is also known from U.K. Patent No. 734,577 to employ aromatic compounds as solvents for absorption of hydrogen sulfide in absorption-stripping operations. Such compounds in this system function only as solvents, and not as catalysts or reactants as no chemical reaction of the hydrogen sulfide normally occurs.
It is known from U.S. Pat. No. 3,956,473 to remove hydrogen sulfide with an organic solvent containing a chelate and/or a transition metal salt in which one of the anions may be bromide. The bromide generally serves no known function in the chelating solution, other than as an anion in stoichiometric amounts to provide the metal as a salt in a soluble form.
It is known from U.S. Pat. No. 4,220,505 to use trihalides(I) to oxidize hydrogen sulfide to elemental sulfur while reducing the trihalide(I) to a zero valent state. The zero valent halide is electrolytically oxidized and thus regenerated for reuse.
It is known from U.S. Pat. No. 4,206,194 that cyanides are formed in the presence of ferrous ions, and that such cyanides destroy anthraquinone disulfonate which is commonly used in the Stretford process.